Conventionally, a seatbelt apparatus mounted on a vehicle, such as an automobile, restrains an occupant with a seatbelt to prevent the occupant from moving forward from the seat in an emergency, such as a vehicle collision. Such a seatbelt apparatus includes a seatbelt retractor that retracts a seatbelt. In the seatbelt retractor, the seatbelt is retracted onto a spool when it is not in use, and is withdrawn and worn by an occupant when it is in use. In the event of an emergency, such as that described above, a means for locking the seatbelt retractor is actuated to prevent the spool from rotating in the belt withdrawing direction, thereby preventing the seatbelt from being withdrawn. Thus, the seatbelt retains the occupant in an emergency.
In conventional seatbelt apparatuses, various belt tension modes are set depending on how the vehicle runs, how the seatbelt apparatus is used, etc. Various seatbelt apparatuses are known which include, as a seatbelt retractor, a motor retractor in which a spool for retracting a seatbelt is rotated by power of a motor. In this seatbelt retractor, to realize belt tension corresponding to a belt tension mode set depending on how the vehicle runs, how the seatbelt apparatus is used, etc., a controller drives and controls an electric motor, serving as a drive means, to control the belt retraction onto the spool and the belt withdrawal from the spool.
For the controller to control the belt retraction onto the spool and the belt withdrawal from the spool by controlling drive of the electric motor, it is necessary to detect the amount and direction of rotation of the spool. A seatbelt retractor has been proposed which includes a rotation sensor that detects the amount and direction of rotation of a spool. Specifically, the rotation sensor includes magnets, a rotating disk supported by a rotating shaft of the spool so as to be rotatable integrally with the spool, and Hall elements (Hall ICs) configured to detect rotation of the rotating disk by detecting the magnets. The seatbelt retractor controls an electric motor on the basis of the amount of rotation of the spool detected by the rotation sensor (see, e.g., Patent Literature (PTL) 1).
FIG. 5 illustrates a seatbelt apparatus described in PTL 1. FIG. 6 illustrates a partial cross section of a seatbelt retractor described in PTL 1. FIG. 7(a) illustrates annularly arranged magnets of a rotation sensor described in PTL 1. FIG. 7(b) is a partial view corresponding to part VIM of FIG. 7(a). FIG. 7(c) is a cross-sectional view taken along line VIIC-VIIC of FIG. 7(b). FIG. 8 illustrates how the rotation sensor detects rotation.
In FIG. 5 to FIG. 8, reference numeral 1 denotes a seatbelt apparatus; reference numeral 2 denotes a vehicle seat; reference numeral 3 denotes a seatbelt retractor configured as a motor retractor; reference numeral 4 denotes a seatbelt withdrawably retracted by the seatbelt retractor 3, the seatbelt having a belt anchor 4a at an end thereof to be secured to a vehicle floor or the vehicle seat 2; reference numeral 5 denotes a guide anchor configured to guide the seatbelt 4 withdrawn from the seatbelt retractor 3 to a shoulder of an occupant; reference numeral 6 denotes a tongue slidably supported by the seatbelt 4 guided by the guide anchor 5; reference numeral 7 denotes a buckle secured to the vehicle floor or the vehicle seat and into which the tongue 6 is engageably and disengageably inserted and engaged; reference numeral 8 denotes an electric motor serving as a drive means that rotates a spool 10 of the seatbelt retractor 3 to retract and withdraw the seatbelt 4; reference numeral 9 denotes a U-shaped frame; reference numeral 9a denotes a left side wall of the frame 9; reference numeral 9b denotes a right side wall of the frame 9; reference numeral 10 denotes a spool configured to retract the seatbelt 4; reference numeral 10a denotes a rotating shaft of the spool 10; reference numeral 11 denotes a lock mechanism 11; reference numeral 12 denotes a deceleration sensing mechanism; reference numeral 13 denotes a spring mechanism serving as a spool retraction biasing mechanism; reference numeral 13a denotes a case of the spring mechanism 13; reference numeral 14 denotes a power transmission mechanism, such as a planetary-gear deceleration mechanism or an external-gear deceleration mechanism; reference numeral 14a denotes a case of the power transmission mechanism 14; reference numeral 15 denotes a rotation sensor serving as a rotation detecting member; reference numeral 16 denotes a controller (CPU); reference numeral 17 denotes a pretensioner; reference numeral 18 denotes a rotating disk of the rotation sensor 15, the rotating disk being mounted through a bushing 10b on the rotating shaft 10a of the spool 10 so as to be concentric with and rotatable integrally with the rotating shaft 10a; reference numeral 19 denotes a bracket secured to the right side wall 9b of the frame 9; reference numeral 20 denotes a pair of Hall elements (Hall ICs) of the rotation sensor 15, the Hall elements being spaced with a predetermined distance therebetween along the circumference of a circle concentric with the rotating shaft 10a, attached to the bracket 19, and electrically connected to the controller 16; reference numeral 21 denotes annularly arranged magnets of the rotating disk 18, the magnets being arranged concentrically with the rotating shaft 10a and including N-pole magnets 21a and S-pole magnets 21b alternately arranged; and reference numeral 22 denotes an annular magnet holding member of the rotating disk 18, the magnet holding member being configured to hold the magnets 21 and mounted on the rotating shaft 10a of the spool 10 so as to be rotatable integrally with the spool 10 and concentric with the rotating shaft 10a. 
When the spool 10 rotates in the seatbelt withdrawing direction, the rotating disk 18 and the magnets 21 also rotate in the seatbelt withdrawing direction. This allows the pair of Hall elements 20 to detect the N-pole magnets 21a and the S-pole magnets 21b and output the resulting detection signals to the controller 16. When the Hall elements 20 detect the N-pole magnets 21a and the S-pole magnets 21b alternately, the current polarities of the detection signals from the Hall elements 20 switch from one to the other. The detection signals from the Hall elements 20 are out of phase by a predetermined amount. The controller 16 counts the number of times the current polarities of the detection signals from the Hall elements 20 have switched, and thereby detects the amount of rotation of the spool 10. On the basis of the amount by which the detection signals from the Hall elements 20 are out of phase, the controller 16 determines whether the spool 10 is rotating in the seatbelt withdrawing or retracting direction. Then, on the basis of the amount and direction of rotation of the spool 10, the controller 16 controls drive of the electric motor 8 to control the belt tension of the seatbelt 4.